Electronic unit injectors

ABSTRACT

An electronic unit injector comprising a spray tip including a valve seat, a needle valve arranged to close on the seat to prevent discharge of fuel from the spray tip or to open off the seat to dispense fuel from the spray tip, a spring biasing the needle valve to a closed position, a spring seat between the spring and the needle valve, the needle valve overcoming the biasing force when the pressure reaches a predetermined level, the spring and seat being disposed in a cage having port areas circumferentially arranged about said spring and spring seat to supply low pressure fuel to the area occupied by said spring and spring seat to reduce the risk of cavitation in said spring cage.

BACKGROUND OF THE INVENTION

The invention relates to improvements in fuel injectors for dieselengines.

PRIOR ART

A common arrangement for diesel injector assemblies has a needle valveimmediately upstream of the injector orifices biased closed by a spring.The needle valve is cyclically opened by an impulse of high pressurefuel operating on an area of the needle valve that opposes the biasingspring. The spring resides in a space, typically in a part of theinjector assembly referred to as a spring cage that is exposed to fuelat low pressure levels. Exposing the spring space to fuel is done toavoid a need and the practical difficulty to completely seal it from thenecessarily high injection pressures. A persistent and seemingly complexproblem in an electronically controlled injector is cavitation in thevalve spring space. This cavitation can lead to degradation of thespring and ultimate failure.

U.S. Pat. No. 6,811,092 is directed to the problem of cavitation in thespring cage of an electronic fuel injector. Experience has shown thesolution proposed in this patent is not effective, at least in certainapplications, in satisfactorily eliminating cavitation in the springcage. The patent indicates an earlier described arrangement of a fuelinjector assembly with a spring cage vented to a low pressure region ofthe injector to avoid a hydraulic lock had a potential for cavitation.

SUMMARY OF THE INVENTION

The invention relates to the discovery that cavitation in a spring cageof an electronic fuel injector can be effectively eliminated byaffording a sufficient, positive supply of fuel to a critical area ofthe spring cage. Where the spring cage, as is conventional, is a hollowcylinder, it has been found effective to port the cage walls with anarea that is at least a significant fraction of the area of the springseat and, preferably, to provide this port area in an arrangementgenerally surrounding the spring seat. Additionally, it is desirable toprovide a port area adjacent the end of the spring cage remote from thespring seat. By porting the spring cage at opposite ends, fuel morereadily circulates in and out of the spring cage area thereby improvingheat transfer, lowering temperature of fuel in the spring cage andreducing the risk of cavitation.

In the disclosed embodiment, the spring cage is arranged to be used withan original equipment manufactured nozzle nut or a duplicate thereof. Assuch, in its preferred embodiment, the spring cage of the invention is ahollow cylindrical body with an outside diameter sized to provide alarge functional clearance with the inside diameter of the surroundingportion of the nozzle nut. The spring cage can be concentrically locatedon the axis of the nozzle nut bore, for example, by indexing it to aspray tip at a lower end and at an upper end to a spacer fitted to thenozzle nut bore. In their assembled state, the spring cage and nozzlenut form an annular fuel plenum surrounding the spring cage which freelycommunicates with all of the ports in the spring cage wall. The annularplenum serves as a local reservoir that can supply fuel and therebyreduce the tendency for cavitation to occur within the spring cage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an injector assembly taken in alongitudinal plane of its central axis;

FIG. 2 is an exploded side view, partially in section, of elements of akit including the novel spring cage (sectional in the planes indicatedat the lines 2-2 in FIG. 3) of the invention for use in the assembly ofFIG. 1;

FIG. 3 is a view of the upper end of the spring cage;

FIG. 4 is a view of the lower end of the spring cage; and

FIG. 5 is a longitudinal cross-sectional view of the spring cage takenin the plane indicated in FIG. 3 at the lines 5-5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An injector assembly 10 for introducing fuel to the cylinder of a dieselengine such as used in a railroad locomotive is illustrated in FIG. 1.The injector assembly 10 is installed on an engine in a known manner.The injector assembly 10 has a general construction like that of theprior art units shown in U.S. Pat. No. 6,811,092, the disclosure ofwhich is incorporated herein by reference. As is common, a separateinjector assembly 10 is provided for each cylinder of the engine.

Most of the components of the injector 10 are centered about an axisindicated at 11. At an upper end, the assembly 10 includes a plungersocket 12 that receives a lever mechanically operated in synchronizationwith the engine's crankshaft. The socket 12 drives a cylindrical plunger13 down into a fuel pressurizing chamber 14 formed in a main body orhousing 16 of the injector 10. A spring 17 encircling the top of theplunger 13 and operating through a retainer 18 returns the plunger fromits fuel pressurizing stroke. Fuel is delivered into the chamber 14 by adistribution rail fed by a fuel supply pump in a known manner. Thesupply pressure of the fuel is relatively low, being typically in therange of about 105 psi. An electronically operated control valve 21 onthe housing 16 is normally open and allows fuel being displaced from thechamber 14 by downward movement of the plunger 13 to be vented at lowpressure to a return circuit. When the control valve 21 is closed byelectrically energizing the coil of its armature, downward movement ofthe plunger 13 is immediately reflected in high pressurization of thefuel remaining in the chamber 14.

The lower end of the cylindrical bore or chamber 14 is closed by acylindrical spacer 22. Below the spacer 22 is a cylindrical spring cage23 and below that is a circular spray tip 24. The spacer 22, spring cage23, and spray tip 24 are held together and against the housing 16 by anozzle nut 26 threaded onto the bottom of the housing. Aligned drilledpassages 27, 28 and 29, through the spacer 22, spring cage 23, andcircular spray tip 24 communicate with one another to deliver fuel fromthe pressure chamber 14 to a cavity 31 in the spray tip. While only onepassage in each of these components is illustrated in FIG. 1, it will beunderstood that two identical passages exist in each of these componentsas is suggested in FIGS. 3 and 4. The angular orientation of the spacer22, spring cage 23, and spray tip 24 relative to one another ismaintained by axially oriented pins 34 received in aligned blind holes35 at their interfaces. A needle valve 36 having a precision sliding fitin a central bore 37 in the spray tip 24 has a tapered end 38 that sealson a seat 39 in the spray tip 24 and controls discharge of fuel out ofthe spray tip through orifices 41 and into a combustion chamber.

The spring cage 23 is a cylindrical tube having an outer cylindricalsurface 46 and an inner cylindrical surface 47 forming a boundary of theinterior space 48 of the spring cage. Assembled in the space 48 are ahelical compression spring 51, a spring seat 52 at the lower end of thespring, and a shim 53 at its upper end. The spring seat 52 has a blindbore in which a reduced diameter stub of the needle valve fits. At itsupper side, the spring seat 52 has a cylindrical shank 54 sized to fitinto the inside diameter of the helical spring 51. When the spray tip24, spring cage 23, and spacer 22 are held in place by the nozzle nut26, the spring 51 is compressed to hold the needle valve 36 closed onthe seat 39 with a predetermined force.

An annular chamber 56, formed between the nozzle nut 26 and body 16receives pressurized fuel from the supply rail, e.g. at about 105 psi.This pressurized fuel communicates with an annular chamber 57 around thespacer through a flat 58 on a threaded area at the bottom of the housing16. Similarly, flats 59 on diametrally opposite outer sides of thespacer communicate rail pressure fuel to the outer periphery of thespring cage 23.

Both the spray tip 24 and spacer 22 have outside diameters that producea close fit with respective surrounding internal surfaces of the nozzlenut 26 so as to hold these elements concentric with the axis 11. Theoutside diameter of the spring cage 23, however, is significantlysmaller than the inside diameter of the respective part of the nozzlenut 26. The axial locating pins 34 serve to hold the spring cageconcentric with the axis 11.

In operation, the plunger 13 is driven downwardly with the forcedeveloped on the socket 12 by the engine's camshaft. Fuel in the chamber14 below the plunger 13 is discharged through a side port in the chamberwall and through an internal passage to the control valve 21 and beyondto a return to the fuel tank. When the control valve 21 closes, fuel inthe chamber 14 is immediately pressurized. This pressure is transmittedthrough the passages 27-29 to the cavity 31. The resulting high fuelpressure in the cavity 31 lifts the needle valve 36 against the force ofthe spring 51 whereupon fuel is injected into the engine cylinderthrough the spray tip orifices 41. A shoulder 64 on an upper end of theneedle valve 36 abuts the spring cage 23 to limit opening movement ofthe needle valve. When the control valve 21 opens, the fuel pressure inthe injector assembly 10 drops, the needle valve 36 closes and injectionstops. This process repeats cyclically as the engine operates.

As a practical matter, pressurized fuel migrates along the needle valve36 from the cavity 31 into the interior space 48 of the spring cage 23.The very rapid movement of the needle valve 36 and the spring seat 52has been found to result in destructive cavitation producing erosion andfailure of the needle valve spring in prior art electronic unitinjectors. With reference to FIGS. 2 and 5, the spring cage 23 has aplurality of ports 61 through its cylindrical wall that have been found,surprisingly, to effectively eliminate cavitation with the spring cageparticularly in the area around the spring seat 52. In one preferredarrangement, the ports 61 are distributed around the circumference ofthe spring cage 23 at four equally spaced locations in a planeperpendicular to the axis 11 and passing through the spring seat shank54. Thus, the ports 61 are at the lower end of the spring cage 23adjacent the spring seat 52. Supplementing these lower ports 61, is atleast one additional port 62 in the spring cage wall adjacent the upperend of the spring 51. It is theorized that the tendency for fuel tocavitate in the area of the spring seat 52 is the result of suddenclosing motion of the needle valve 36 caused by the requisite high forceapplied by the spring when the pressure in the cavity 31 drops followingopening of the control valve 21. This jerk-like motion of the springseat 52 requires a similar movement of fuel directly behind it. Bylocating the ports 61 at or adjacent the plane of the spring seat 52 andmaintaining the fuel at these ports above atmospheric pressure, i.e. atthe level of the fuel supply rail, it is believed that a sufficientquantity of fuel at a sufficient positive pressure is maintained behindthe space vacated by the spring seat as it drives the needle valveclosed. An annular space 60 between the nozzle nut 26 and spring cage 23serves as a fuel reservoir to instantaneously feed fuel to the space 48or interior of the spring cage 23 through the ports 61 should alocalized low pressure condition occur behind the spring seat 52 as thespring 51 snaps the needle valve 36 closed. A factor in effectiveavoidance of cavitation is the collective cross-sectional area of theports 61 being at least a significant fraction of the cross-sectionalarea of the spring seat 52. In the illustrated arrangement, the springseat 52 has a nominal diameter of 0.392″ and the collective area of theports 61 is at least about ¼ the cross-sectional area of the springseat. Further, the ID of the nozzle nut is nominally 0.965″ and the ODof the spring cage is nominally 0.933″ leaving a cross-sectional area ofthe reservoir space between these surfaces approximately 4/10 of thearea of the spring seat 52. The upper port 62 can have the same diameteras that of the lower ports 61. The reciprocating motion of the springseat 52 as it follows the motion of the needle valve 36 can inducecurrents in the fuel in the spring cage 23 through the ports 61, 62 withthe result of an improvement in heat transfer, thereby reducingtemperature and, therefore, the risk of cavitation of fuel in the springcage.

It should be evident that this disclosure is by way of example and thatvarious changes may be made by adding, modifying or eliminating detailswithout departing from the fair scope of the teaching contained in thisdisclosure. The invention is therefore not limited to particular detailsof this disclosure except to the extent that the following claims arenecessarily so limited.

1. An electronic unit injector comprising a housing with a fuel pressurechamber, a mechanically operated plunger reciprocating in the housingchamber to pressurize fuel in the chamber, an electronically controlledvalve for venting fuel from the chamber when it is open and therebypreventing high pressurization of fuel in the chamber by the plunger andallowing the plunger to pressurize fuel in the chamber to an injectionpressure when it is closed, a spray tip including a valve seatcommunicating with the fuel pressurization chamber, a needle valvearranged to close on the seat to prevent discharge of fuel from thespray tip or to open off the seat to dispense fuel from the spray tip, aspring biasing the needle valve to a closed position on the valve seat,a spring seat disposed between the spring and the needle valve, theneedle valve being arranged to overcome the biasing force of the springwhen the pressure in the pressure chamber reaches a predetermined levelas a result of the electronically controlled valve closing, the springand seat being disposed in a cage, the cage having a port areacircumferentially arranged about said spring and said spring seat toallow fuel from a low pressure source to supply fuel to the areaoccupied by said spring and spring seat to reduce the risk of cavitationin said spring cage, said cage having a cylindrical wall and said portarea comprising a series of radial holes distributed about thecircumference of said cylindrical wall, the collective area of saidholes being at least about one-quarter of the area of the spring seat.2. An electronic unit injector comprising a housing with a fuel pressurechamber, a mechanically operated plunger reciprocating in the housingchamber to pressurize fuel in the chamber, an electronically controlledvalve for venting fuel from the chamber when it is open and therebypreventing high pressurization of fuel in the chamber by the plunger andallowing the plunger to pressurize fuel in the chamber to an injectionpressure when it is closed, a spray tip including a valve seatcommunicating with the fuel pressurization chamber, a needle valvearranged to close on the seat to prevent discharge of fuel from thespray tip or to open off the seat to dispense fuel from the spray tip, aspring biasing the needle valve to a closed position on the valve seat,a spring seat disposed between the spring and the needle valve, theneedle valve being arranged to overcome the biasing force of the springwhen the pressure in the pressure chamber reaches a predetermined levelas a result of the electronically controlled valve closing, the springand seat being disposed in a cage, the cage having a port areacircumferentially arranged about said spring and said spring seat toallow fuel from a low pressure source to supply fuel to the areaoccupied by said spring and spring seat to reduce the risk of cavitationin said spring cage, said spring cage having port areas adjacent saidspring seat and remote from said spring seat whereby circulation of fuelthrough said spring cage can be induced by movement of said spring seat.3. An electronic unit injector comprising a housing with a fuel pressurechamber, a mechanically operated plunger reciprocating in the housingchamber to pressurize fuel in the chamber, an electronically controlledvalve for venting fuel from the chamber when it is open and therebypreventing high pressurization of fuel in the chamber by the plunger andallowing the plunger to pressurize fuel in the chamber to an injectionpressure when it is closed, a spray tip including a valve seatcommunicating with the fuel pressurization chamber, a needle valvearranged to close on the seat to prevent discharge of fuel from thespray tip or to open off the seat to dispense fuel from the spray tip, aspring biasing the needle valve to a closed position on the valve seat,a spring seat disposed between the spring and the needle valve, theneedle valve being arranged to overcome the biasing force of the springwhen the pressure in the pressure chamber reaches a predetermined levelas a result of the electronically controlled valve closing, the springand seat being disposed in a cage, the cage having a port areacircumferentially arranged about said spring and said spring seat toallow fuel from a low pressure source to supply fuel to the areaoccupied by said spring and spring seat to reduce the risk of cavitationin said spring cage, said cage having a cylindrical wall and said portarea comprising a series of radial holes distributed about thecircumference of said cylindrical wall, said spring cage beingsurrounded by an annular space having a cross-sectional area of at leastone-quarter of an area of the spring seat.
 4. A kit for use in anelectronic unit injector comprising a circular spray tip having a majordiameter outer surface, a valve seat, and a bore concentrically arrangedabout an axis, a needle valve receivable in said bore with a precisionsliding fit, a spring cage adapted to abut a rear face of the spray tipand limit opening movement of the needle valve, the spring cage beingformed by a circular tubular wall, a shim, a spring, and a spring seatreceivable in the spring cage, the spring cage and spray tip beingadapted to be retained in an operating position, with the needle valvein the spray tip and the spring cage, and with the spring seat, springand shim in the spring cage, by a nozzle nut threaded with an injectorbody, the spring cage being smaller in diameter than the spray tip, thespring cage having a port area formed by a plurality ofcircumferentially disposed ports in its wall for permitting freecommunication of fuel between the outside and inside of the spring cagewhereby the risk of cavitation in the spring cage is reduced.
 5. A kitas set forth in claim 4, wherein said ports are axially proximate tosaid spring seat when the kit is in an assembled state.
 6. A kit as setforth in claim 5, wherein said port area includes a location remote fromsaid spring seat so as to provide circulation of fuel through saidspring cage as a result of movement of said spring seat during openingand closing movement of said needle valve.